First. Romney's mormonism is not a factor. He is not going to govern as a mormon.
I'm guessing Romney's mormonism would only be "offensive" to some evangelicals (because of how seriously they take Christianity, and how offended they might be with the LDS conception of Christ).
The GOP is not worried about its evangelicals because they have built a very powerful message system. Remember: the GOP bussed medicare recipients to Town Hall meetings to complain about government controlled health care. They were also able to morph Bin Laden into Hussein without the slightest worry that their base knew the region well enough to question their narrative. They turned Bod Dole's family counseling health care provision into a "death panel" with zero fear that the base would notice. Mitt Romney created ObamaCare before Obama. The GOP can run whoever it wants, and like Bush, that person can spend twice as much as his Democratic predecessor without the worry of one Tea Party rally. Their base knows nothing of the largest Amnesty Bill ever passed (by Reagan, which flooded California and the Southwest with job-stealing labor). The GOP base knows nothing of Iran-Contra and the selling of weapons to the world's leading terrorist nation. They know nothing of our relationship with the Shaw or mujihadeen. The GOP voter lives in a bubble constructed by a 30 year investment in think tanks and media. As a result, the talk radio voter is utterly dependent upon the GOP for information and analysis. (It's ironic that these anti-government voters get all their information from a political party)
My point: If GOP media chooses not to focus on Mitt's religion, than his religion literally won't exist. Fortunately, in this case, I'm glad it won't exist.